<!DOCTYPE html>
<html>

<head>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <style>
        #myDIV {
            width: 100%;
            padding: 50px 0;
            text-align: center;
            background-color: lightblue;
            margin-top: 20px;
            display: none;
        }
    </style>
</head>

<body>

    <p>Click the button to find out if the DIV is hidden. If it is, then display it.</p>

    <button onclick="myFunction()" style="font-size: 55px;">Try it</button>

    <div id="myDIV">
        This is my DIV element.
    </div>

    <p>Some text...</p>

    <pre>
    <h2>
    检测一个元素是否隐藏了:
    var x = document.getElementById("myDIV");
    if (window.getComputedStyle(x).display === "none") {
        x.style.display = "block";
    }

    或:

    var x = document.getElementById("myDIV");
    if (window.getComputedStyle(x).visibility === "hidden") {
      // Do something..
    }

    </h2>
    </pre>

    <script>
        function myFunction() {
            var x = document.getElementById("myDIV");
            if (window.getComputedStyle(x).display === "none") {
                x.style.display = "block";
            }
        }
    </script>

</body>

</html>